This invention relates to a process which effectively removes impurities such as water vapor and CO.sub.2 from commonly available compressed air through the use of gas separation membranes which provides an inexpensive source of gas needed for operation of analytical instrumentation such as Fourier Transform Infrared analytical spectrometers (FTIR). Such instruments need high quality gas for purging of the instrument optical compartments to insure the absence of impurities, such as water vapor and CO.sub.2. These impurities cause serious problems when they are present at concentrations typical of ambient air or of commonly available compressed air utility sources in many laboratories. For example, FTIR instruments are used for analyses of samples by examination of their absorption spectra in the infrared wavelength region of the electromagnetic spectrum. Such instruments employ sensitive optical elements, such as salt crystals made of highly moisture sensitive materials. The presence of even modest levels of moisture degrades such optical components severely. Further, impurities such as water vapor and CO.sub.2 are strong absorbers of infrared radiation, at wavelengths which interfere with analytical wavelengths and spectral regions commonly employed for analysis of a variety of organic and inorganic materials. Such strong interfering infrared absorptions, in addition to obscuring the spectra of samples, reduce instrument and analytical sensitivity by reducing the radiant energy intensity of the instrument's infrared optical source radiation by strongly absorbing the source energy before it can pass through the analytical sample.
Modern instrumentation such as the FTIR spectrometers operate on the principal of an interferometer and employ a moving mirror in the optical train in routine operation. Such moving mirror elements execute motion of the moving mirror element by floating of the mirror's piston drive on a gas bearing to eliminate friction and vibration effects. Gas employed to float and activate such gas bearings actually exhausts into the instrument's optical compartment and thus must be of high quality and free of contaminating impurities. Presently, users of such analytical instruments can obtain operating gases, either nitrogen gas or clean air from sources such as purified tank gas, liquid nitrogen boil off, or pressure swing desiccant treated compressed air systems. All of these sources have limitations in terms of convenience, cost or practical operational limitations.
Purified compressed tank supplies of gas, for example, are expensive due to the high volumes of purge and operating gas routinely employed in instrument operation. Such high volumes of gas used also result in inconvenience to the operator in frequency of tank replacement and connection which also increases the risk of introduction of impurities from the ambient environment during disconnect/reconnect procedures.
Pure nitrogen gas supplied from typical lab utility sources such as from liquid nitrogen boil off is commonly available at pressures which are only marginally adequate for operation of the gas bearing for the moving mirror optical elements, such as described for FTIR spectrometers. Compression of such liquid nitrogen boil off to higher pressures is possible, but entails the risk of the addition of impurities in such compressor systems. Commonly available utility nitrogen from boil off is often at pressure of about 30 psig to 35 psig (4.35-5.08.times.10.sup.3 Pa). Many moving mirror optical elements in modern FTIR spectrometers require a minimum pressure of about 28-30 psig (4.06-4.35.times.10.sup.3 Pa) to effectively float the gas bearing of the moving piston drive mechanism. When multiple supply taps from the same nitrogen source are utilized at different times, the line pressure frequently falls, at least temporarily below that needed to float the FTIR gas bearing, thus interrupting and compromising routine instrument operation and useability.
Several commercially available apparatus permit the production of high quality instrument operating gas by removal of water vapor and CO.sub.2 from commonly available compressed air, where such units employ absorption desiccant beds to effect such impurity removal. Such desiccant units operate using two beds of a solid absorbent or adsorbent material operating under the principle of cyclic pressure swing processes. These units typically employ one desiccant bed online while a second bed undergoes cyclic regeneration. Units cycle typically every few minutes, involving substantial noise and vibration each time a bed is depressurized and require relatively frequent recharge of the sorbent solid material, especially those which remove carbon dioxide. These units create considerable nuisance and operational inconvenience and expense for the user.